Question

One difficulty with the original TCP SRTT estimator is the choice of an initial value. In the absence of any special knowledge of network conditions, the typical approach is to pick an arbitrary value, such as 4 seconds, and hope this will converge quickly to an accurate value. If this estimate is too small, TCP will perform unnecessary retransmissions. If it is too large, TCP will wait a long time before retransmitting if the first segment is lost. Also, the convergence might be slow.

a. Choose α=0.8 and SRTT(0)-1 seconds, and assume all measured RTT values-0.6 second with no packet loss. What is SRTT(12)? Recall,
SRTT(k + 1)-α . SRTT(k) + (1-a) . RTT(k + 1).
Describe your solution approach AND provide the numerical result (approximate to the 4th decimal place)
b. Using the same values as in part a), what happens if we use α-07 or α-09? Provide a numerical result for SRTT(12) in both cases, then describe the effect of a larger or smaller a on the RTT estimation procedure
c. What is the retransmission ambiguity problem addressed by the Karn-Partridge algorithm? How does the algorithm avoid the ambiguity?

Answer 1

The TCP SRTT estimator estimates the round-trip time for packet transmission. The initial value is difficult to choose, but an arbitrary value can be used. The SRTT(12) value can be calculated using the given formula and values.

Step-by-step explanation:

The SRTT (Smoothed Round-Trip Time) estimator in TCP (Transmission Control Protocol) is used to estimate the time it takes for a packet to travel from the sender to the receiver and back. The Karn-Partridge algorithm addresses the retransmission ambiguity problem by updating the RTT estimate only when a non-retransmitted packet is acknowledged.

One difficulty with the original TCP SRTT estimator is the choice of an initial value. The typical approach is to pick an arbitrary value and hope it converges quickly to an accurate value. In this case, α is chosen as 0.8, SRTT(0) is chosen as 1 second, and all measured RTT values are 0.6 second with no packet loss.

To calculate SRTT(12), we can use the formula: SRTT(k + 1) = α * SRTT(k) + (1 - α) * RTT(k + 1).

Plugging in the values, we get: SRTT(12) = 0.8 * (SRTT(11)) + 0.2 * 0.6 = 0.8 * SRTT(11) + 0.12.

For part b, if we use α = 0.7, the calculation for SRTT(12) would be slightly different, and if we use α = 0.9, it would be even more different. The effect of a larger α is that it weights the most recent RTT value more heavily, resulting in faster convergence to changes in network conditions.

The retransmission ambiguity problem addressed by the Karn-Partridge algorithm is the difficulty in accurately estimating the round-trip time (RTT) in TCP when retransmissions occur. The algorithm avoids ambiguity by only updating the RTT estimate when a non-retransmitted packet is acknowledged. This ensures that the RTT estimate is based on packets that have not experienced retransmissions.